1. Field of the Invention
The present invention relates to a radiation treatment planning system and a method of radiation treatment planning.
2. Description of the Related Art
Radiation treatment involves irradiating the tumor cells constituting the target with radiation for treatment. Whereas X-rays are the most commonly employed radiation in radiation treatment, demand has been growing for treatments that use particle beams (charged particle beams) typified by proton beam and carbon ion beam offering high dose concentration on the target.
In radiation treatment, excessive or insufficient doses of irradiation can lead to adverse effects on normal tissues other than the tumor or a relapse of the tumor. It is thus required that the tumor region be irradiated with a dose as accurate as possible and as concentrated as possible.
An X-ray-based therapy called IMRT (Intensity Modulated Radiation Therapy) involves giving irradiation in multiple directions while varying the collimator shape. Where the target region of a complicated shape is irradiated, this therapy can minimize the dose with which the surrounding normal tissues are irradiated. In the treatment using particle beams, numerous fine beams with their intensities modulated are applied at a plurality of irradiation angles, which is known as IMPT (Intensity Modulated Proton Therapy) whereby a uniform dose distribution is provided over the target region.
IMPT is implemented using the scanning irradiation method. The scanning irradiation method involves causing a pair of scanning magnets to deflect fine charged particle beams to a desired point on a plane. The method allows the inside of the tumor to be irradiated in a filling manner so that a high dose is applied only to the tumor region. With this method, diverse dose distributions can be formed easily.
With IMRT and IMPT, what is very important is the process of preparing the treatment plan using a radiation treatment planning system before actual irradiation. The radiation treatment planning system simulates by numerical calculation the dose distribution inside the patient's body based on the information about the inside of the patient's body obtained from computed tomography images or the like. The operator of the radiation treatment planning system determines irradiation conditions such as the direction and energy in which radiation is applied, the irradiation point and the irradiation amount, by referring to the results of the calculations performed by the apparatus. The generally practiced process in this regard is outlined below.
First, the operator inputs the target region to be irradiated with radiation. What is input here is the region targeted to be sliced into images mainly using CT images. When the operator registers the input data with the radiation treatment planning system, the data is stored as three-dimensional region data into a memory of the radiation treatment planning system. If necessary, the operator may also input and register the position of organ at risk.
The operator then sets the prescription doses constituting the target doses for the registered regions. The settings are made for the target region and the organ at risk, which are registered earlier. With regard to the target region, for example, a dose sufficient for necrotizing the tumor is designated. In many cases, a minimum value and a maximum value of the dose with which to irradiate are designated for the target region. On the other hand, a maximum permissible dose is designated regarding the organ at risk.
It is the radiation treatment planning system that determines the beam irradiation point and irradiation amount for implementing the dose distribution designated by the operator. Usually, the irradiation point is determined first. Thereafter, the irradiation amount is determined in a manner that meets the dose distribution conditions input by the operator.
A widely adopted method for efficiently determining the irradiation amount involves using an objective function that quantities the divergence from the prescription dose, as described by A. Lomax, in “Intensity modulation methods for proton radiotherapy,” Phys. Med. Biol. 44 (1999), 185-205 (Non-Patent Literature 1). The objective function is defined to be smaller the more closely the dose distribution meets the prescription dose. A search is made for an irradiation amount that minimizes the objective function through iterative calculation, whereby an optimum irradiation amount is calculated.
As a result, it is possible to calculate the irradiation amount for obtaining the dose distribution that satisfies the prescription dose. However, there exist numerous uncertainties such as those in setup the patient (setup error), in letting the target be moved by respiration (respiratory movement error), and in converting CT values to water equivalent thickness (range error), as described by A. Lomax, in “Intensity modulated proton therapy and its sensitivity to treatment uncertainties 1: the potential effects of calculation uncertainties,” Phys. Med. Biol. 53 (2008), 1027-1042 (Non-Patent Literature 2) and also by A. Lomax, in “Intensity modulated proton therapy and its sensitivity to treatment uncertainties 2: the potential effects of inter-fraction and inter-field motions,” Phys. Med. Biol. 53 (2008), 1043-1056 (Non-Patent Literature 3).
Given the above-mentioned uncertainties, there is a possibility that the dose distribution inside the patient's body under irradiation may not match the dose distribution of the prepared treatment plan. Particularly with IMRT and IMPT, the effects of these uncertainties can become prominent because a desired dose distribution is formed by preparing and adding up uneven dose distributions applied in a plurality of irradiation directions.
In order to alleviate the effects of such uncertainties upon preparation of a treatment plan, there have been proposed several techniques that focus attention on characteristics other than the divergence between the dose of each region (target region and organ at risk) and the prescription dose. For example, according to one technique, on the assumption that the range of the beam applied to the target is varied randomly, optimized calculation is performed of the values of the dose provided by the beam and of the objective function so as to lower the sensitivity of the dose distribution in the face of range uncertainties, as described by J. Unkelbach et al., in “Accounting for range uncertainties in the optimization of intensity modulated proton therapy,” Phys. Med. Biol. 52 (2007), 2755-2773 (Non-Patent Literature 4). According to another technique, as described by D. Pflugfelder et al., in “Worst case optimization: a method to account for uncertainties in the optimization of intensity modulated proton therapy,” Phys. Med. Biol. 53 (2008), 1689-1700 (Non-Patent Literature 5), the dose distribution in effect when a setup error has occurred in addition to the range error is calculated under a plurality of conditions. Information about a maximum divergence between the calculated dose of each region and the prescription dose is added to the objective function as another item for optimized calculation. When the range error and setup error take place, this technique makes it possible to reduce the maximum divergence between the dose of each region and the prescription dose and thereby lower the sensitivity of the dose distribution in the face of range and setup uncertainties.